1. Field of the Invention
This invention relates to distance measuring apparatus and, more particularly, to a micrometer system and a process for its use.
2. Discussion of the Prior Art
A micrometer measures distance between two points on an object. Heretofore, manually operated micrometers have required considerable skill, experience and practice in order to accurately measure specimens. In addition, reading a micrometer can be difficult when it is necessary to manually record the measured value. Accordingly, the use of manually operated micrometers is error prone.
In manufacturing operations, it is often necessary to make many hundreds of precise micrometer measurements on a device being serviced or studied. Two common forms of micrometers are the outside type (with the familiar "P" shaped frame) and the inside micrometer (essentially a rod of accurately known length with an extendible micrometer spindle fitted to the end). One part of the micrometer is fixed and the other part can be made to move toward or away from the fixed part through use of a manually rotated barrel that advances or retracts a threaded section. Each revolution of the barrel usually advances or retracts the moveable part by a predetermined amount. In standard non-metric micrometers, a common increment is 0.025 inches per revolution. Engraved markings on the micrometer handle enable the user to determine how far the parts have opened or closed.
A variation of the micrometer is the vernier caliper. In this device, one jaw is made to slide toward or away from a fixed jaw and an accurately engraved linear scale is read to measure the jaw opening.
To illustrate the difficulties of manual measurement, consider the task of measuring the diameter of a rod. The operator uses an outside micrometer and opens the parts sufficiently to clear the rod, inserts the rod crosswise, and then rotates the micrometer barrel to close the parts to a position where the parts just contact the surface of the rod. In addition, the operator must ensure that the micrometer is applied to the widest point of the rod and is held perpendicular to the axis of the rod. An inside micrometer would be used to measure the diameter of a hole in a similar way.
If both the object being measured and the micrometer frame are rigid or noncompliant, then the measurement process of advancing the micrometer parts until they "just touch" the surface is straightforward. As soon as the parts touch the surface, it becomes impossible to advance the moveable part any further. If (as is usually the case) the micrometer frame or the object surface has some compliance, then considerable skill (or "touch") is required to determine that the parts are "just touching" the surface but not deforming the surface.
One particular situation arises frequently when making measurements on large objects or machines such as steam turbines and electric generators. For apparatus of this type, a "tight wire" measurement is often used. A thin wire is tightly stretched across some part of the object, usually in a way that the ends of the wire touch the object with the center of the wire being unsupported. The wire thus forms a reference datum line against which specified distances are to be measured. The wire always has a fair amount of compliance since it is only possible to tighten the wire a limited amount, before it snaps.
Using an inside micrometer which typically includes a fixed segment and an adjustable segment, the operator selects an appropriate fixed segment from a set of rod-like threaded sections of accurately known length. The operator then assembles the fixed and adjustable segments together to form a rod of known fixed length, slightly shorter that the length to be measured. The adjustable section includes a micrometer head or barrel and adjustable spindle (with perhaps 0-2 inches of travel). The micrometer head or "head" is screwed onto the fixed section, yielding a device to measure length from L to L+2 inches. L can be in the range of 10-15 feet.
The fixed end of the micrometer is placed against a portion of the object, for example, the frame of a turbine, and the movable end is positioned near the tight wire. The operator positions the moveable end of the inside micrometer so that it "just touches" the wire. This cannot be done by touch since the wire is very compliant. It is often impossible to detect the displacement of a wire until a large error (e.g., 0.020-0.040 inches) has occurred. To overcome this problem, an electrical circuit is often employed that generates a tone when the metal micrometer tip touches the metal wire and thus completes an electrical circuit. Sometimes a visual indication (e.g., a lamp or light emitting diode) is employed instead of a tone. By slowly advancing the micrometer tip while sweeping the tip back and forth in a small arc, the operator just barely causes electrical contact to be made. The micrometer is then read by the operator. This procedure must be repeated a number of times to be sure that the minimum distance has been measured. The process is thus both cumbersome and error prone.
There is, therefore, need for a device that can automatically advance the micrometer tip in such a way that the tip is made to "just touch" the tight wire. When the micrometer "just touches" the wire, the device should electronically "read" and store the micrometer reading. Such device also should automatically repeat the cycle a number of times to allow the manually held micrometer to be positioned so as to obtain a true reading. The device should then transfer the reading to a computer, if desired.